Monosilane is present in a gas state with a boiling point of 112° C. under general conditions. Monosilane is actively decomposed at a temperature of 700° C. or more into silicon, which is a basis for obtaining high-purity silicon, and hydrogen.
A series of methods of synthesizing monosilane have been widely known. For example, as a catalytic disproportionation method of trichlorosilane, a two-step technology is known (German Patent No. 3311650 (Oct. 13, 1983). In the technology, the first step corresponds to performing catalytic hydrogenation of metallic silicon, and at this time, the hydrogenation is conducted along with production of trichlorosilane at a temperature from 400° C. to 600° C. and a pressure from 0.7 bar to 41.4 bar. In the second step, a catalytic disproportionation reaction of trichlorosilane is performed. At this time, an anion exchange resin is used as a catalyst along with tertiary amine, and the disproportionation is performed at a maximum temperature of 150° C. Thereafter, impurities of monosilane are removed. However, when a corrosive chlorine compound is present, the invention makes it difficult to purify monosilane into high-purity monosilane due to transfer of impurities from the wall of equipment toward monosilane and causes an increase in the price of a final product, and thus it is difficult to widely use the method to prepare monosilane.
Further, as a method of preparing silane using reaction of silicon tetrachloride with lithium hydride, a method according to Russian Patent No. 2173297 (2001) is also widely known. In the method, lithium hydride is pulverized beforehand, and then the process is conducted at a temperature of 300° C. Lithium chloride produced during the reaction is used for regeneration of LiH. At this time, hydrogen chloride is produced. A major disadvantage of the method is that it is necessary to remove a starting reagent from silane, particularly, hydrogen chloride residue. In relation to this, there is a problem in that it is required to additionally perform a difficult process of removing HCl from a final product.
As another method of preparing silane, there is a method of preparing silane by treating lithium silicide with a dilute solution of hydrochloric acid, nitric acid or sulfuric acid according to Russian Patent No. 2194009 (2002). As a major disadvantage of the method, there is a problem in that it is necessary to additionally perform a difficult process of removing a starting reagent, that is, hydrogen chloride or the corresponding acetic acid or sulfuric acid residue from silane. Examples of other limitations include high exothermicity, and when a violation occurs in the replenishment mode of a starting reagent in a reactor, the reactor is likely to explode.
A method of preparing monosilane using catalytic disproportionation of alkoxysilane is one of the promising methods. The method was proposed in U.S. Pat. No. 2,530,367 in 1947, and the disproportionation in the invention is performed by the following Reaction Equation A.4SiH(C2H5O)3→SiH4+3Si(C2H5O)4  [Reaction Equation A]
The foundation for the catalytic disproportionation of alkoxysilane was established in the former Soviet Union in 1957 to 1959, and the reason for this appears to be that triethoxysilane was usually developed at that time [Soviet Journal of Technical Physics, 1957, v. 27, No. 8, pp. 1645-1648 and Soviet Journal of Solid State Physics, 1959, v. 1, pp. 999-1001].
U.S. Pat. No. 3,829,555 (Aug. 13, 1974) proposed a continuous synthesis process of monosilane. In the invention, sodium ethoxide in tetraethoxysilane acts as a catalyst in a disproportionation reaction of triethoxysilane, the reaction was conducted in a liquid phase, and the conversion ratio of triethoxysilane ranged from 70% to 90%.